1. Field of the Invention
The present invention relates to a process for removing sulfate ions from extracted phosphoric acid.
2. Description of the Prior Art
In the past, it has been known that wet process phosphoric acid which is produced by sulfuric acid decomposition of phosphate rock, can be extracted by an organic solvent which dissolves phosphoric acid and has low solubility to water. Such solvents include various alcohols, ketones, ethers, phosphoric acid esters, amines, and the like. After this treatment, the phosphoric acid is extracted by water from the extracted solution of phosphoric acid (hereinafter referred to as the "extraction method"). In this specification, phosphoric acid prepared by purifying the crude phosphoric acid from the wet process (including a pretreated acid) by the extraction method (including a concentration step) will be referred to as the "extracted phosphoric acid".
The phosphoric acid produced by the wet process contains various metallic ion impurities derived from a phosphate rock and sulfate ions derived from sulfuric acid. Using the extraction method, the metallic ion impurities can be effectively removed. However, the sulfate ions are not satisfactorily removed and a large amount of sulfate ions still remain in the extracted phosphoric acid. The phosphoric acid used in industrial products, medicines and foods must be a pure phosphoric acid having substantially no sulfate ions, such as one having less than 30 wt. ppm as SO.sub.4 as in the Japanese Industrial Standard. In order to decrease sulfate ions sufficiently for use of the phosphoric acid in the industrial applications, it is necessary to remove sulfate ions from the wet process phosphoric acid or from the extracted phosphoric acid. It has been known to remove sulfate ions as calcium sulfate from a wet process phosphoric acid by adding a calcium compound such as phosphate rock, calcium hydroxide or calcium carbonate. However, calcium sulfate has a relatively high solubility. Accordingly, even though a large excess of the calcium compound is used, sulfate ions remain present in a concentration in the range of 1000-2000 wt. ppm as SO.sub.4. It is difficult to lower the concentration of the sulfate ions.
Even when the desulfated wet process phosphoric acid is further purified by extraction, sulfate ions in a concentration of at least 500 wt. ppm as SO.sub.4 remain in the extracted phosphoric acid. It has also been known to remove sulfate ions as barium sulfate and thereby lower the concentration by using a barium compound. However, it is not preferred to use that method on the wet process phosphoric acid because of the following reasons.
a. The sulfate ion content is higher than that of the extracted phosphoric acid whereby the consumption of the expensive barium compound is high. PA1 b. Hydrofluorosilicic acid is generally present whereby the barium compound is consumed for the formation of crystals of barium fluorosilicate. PA1 c. The concentrations of the sulfate ion and hydrofluorosilicic acid fluctuates depending upon the fluctuations in the composition of the phosphate rock and the operating conditions are critical whereby the phosphoric acid may be contaminated by barium ions because of an inadvertant excess addition of the barium compound. PA1 d. Hydrofluoric acid is also present. Accordingly, the selection of a suitable material for the filter used for separating the barium sulfate is restricted. Moreover, it is difficult to use stainless steel. PA1 e. The crystals of barium sulfate are fine and smaller than those of the extracted phosphoric acid. This may be caused by the presence of organic materials and metallic impurities. PA1 1. The dissolved barium ions are adsorbed on the active carbon. PA1 2. The aging time is shortened by the adsorption of the barium ions. PA1 3. The sulfate ions are further adsorbed on the active carbon to a high degree by the adsorption of barium ions.
Moreover, the viscosity of the slurry is high even though the concentration of phosphoric acid is low. Accordingly, the separation of barium sulfate is difficult.